Apparatus for coating



March 14, 1944. J. CAMPBELL EI'AL 2,344,232

' APPARATUS FOR COATING Original Filed Dec. 12, 1940 cur/1v aunt/r10agrouwarn/Ala mum/c comm-mow "rm Merge/1v INVENTOR Jo/w annual I Iawn/0n #707108 v 5 Patented Mar. 14, 1944 APPARATUS roa COATING JohnCampbell and Kenneth Austin Taylor, Glens Falls, N. Y., assignors toInternational Paper Company, a corporation of New York Originalapplication December 12, 1940, Serial No. 369,778. Divided and thisapplication August 12. 1942, Serial No. 454,496

team.

This invention relates to new and useful improvements in coating andparticularly seeks to provide. a novel method and apparatus for applyinga smooth uniform mineral coating to a continuously moving web of paperor the like.

This application is a division of our co-pending a'pplication, SerialNo. 369,778, filed December 12, 1940.

One procedure which may be employed in coating a web of paper is,broadly, to apply the coating composition to the web in greaterquantities than required to produce the desired weight of coating andthen remove the excess through' paper, considerable difliculty has beenen-- countered as a result of the formation of a multiplicity ofimperfections in the applied coating. These imperfections were generallycaused as a result of the passage of a moving web of paper to which anexcess of coating composition had been applied over some form ofsmoothing or wiping device such as a. doctor blade, a smoothing roll, orother device, the web-contacting surfaces of which have been wettable bythe coating composition. Once the web contacting surfaces have becomewet, the aflinity of the coating material for the wetted surface tendsto cause a splitting of the coating film between the web and through theinclusion of several new factors which have heretofore been given littleif any consideration.

A doctor blade constructed in accordance with this invention togetherwith the method of use thereof facilitates the application of a smooth.uniform coating to a traveling web of paper with substantiallyno'surface imperfections whatsoever. This desirable result has beenachieved through providing a smoothing device such as a doctor bladewhich includes a web-contacting surface provided with a layer ofnon-wettable material whereby passage of the coated web thereover forsmoothing purposes will not cause a splitting of the coating such as hasbeen hereto fore encountered with the prior known devices.

One manner ofproviding a non-wetting surface the smoothing device whichresults'in marked irregularities in the finally coated surface.

, Heretofore those doctor blades which have been employed with varyingdegrees of success to remove the excess coating from the web have beenshaped to provide either a cylindrical surface disposed in contact withthe web or have been provided with web-contacting surfaces ofprogressively decreasing radii of curvature. Each of these prior knowntypes depends for its effectiveness upon the combination of a specialshape of the web-contacting portion together with a generallydimensionally restricted maximum radius of curvature terminating in aminor radius of curvature at substantially the point at which the webleaves the doctor blade. In many instances where these prior knowndevices have been used, a small-pool of coating composition has formedadjacent the doctor blade but on the trailing side thereof whichfrequently caused stippling or streaking of the applied coating.

A doctor blade constructed in accordance with the present inventionavoids the diiiiculties heretofore encountered with prior known deviceson the doctor blade is to coat the outer surface of the doctor bladeincluding the web-contacting portion thereof with a material such as acomposite blend of waxes and resins which possesses "non-wettingcharacteristics, or by electro-depositing on the surface of the blade alayer of metal such as chromium or the like. 'If the websuflicient tominimize abrasion. If the web-contacting surface of the blade isprovided with a plating of chromium or like metal, the plated surfaceshould be highly polished with'oil bearing polishing agents in order toprovide both the requisite smoothness to prevent abrasion and to providethe desired non-wetting characteristics. In this connection it should benoted that by employing oil bearing agents to polish the plating, or byemploying some form of oil such as mineral oil subsequent to the actualpolishing of the plating, the pores of the plating will become oilimpregnated to thereby produce a surface having a low afllnity for andimmiscible in those aqueous mineral coating compositions in conjunctionwith which the doctor blade is to be used.

Therefore it is an object of this invention to provide a doctor bladefor removing an excess of coating composition from a continuously movingweb and which includes a convex web-contacting portion on its leadingside and terminating in an intersection with a plane surface disposednormal to the tangent at the intersection. Another object of thisinvention is to provide a device of the character statedwhich includes aweb-contacting portion having a constant radius of curvature, insection, on its leading side 'b epreventedoithcc'oatin! ccmpositionoverbetween Another object of this invention is to-provide a is't provideamassdeviceofthecharacterstatedinivhichthehdnwetfll'lgcharacteristicsazeobtainedasaresultoiproviding the web-contactingysurfaces with a layer of smoothly finishednon-wetting material. Another object of this inycntion is to provide adevice of thecharactenstated' in which thenon-wettingchara'cteristicsareobtainedasa re-.-niltotelectro-depos'itingalayer oi'metal su h l chromiumorthelikeonthe'surfaceoi'.thedoc-' torbisde and then highly layer withoil-bearing P lishing i ts.

A further object of this invention is to provide a doctor blade-of thecharacter described which issimple in dcsisn. tuned in construction and4 economical to manufacture.-

With these and other objects in view, the nature of which willbecomemore apparent. the in-* vention will be more-tony understood byreference to thcdrawing, the accomll rins detail dwcription, and theappended claims.

i ,In thevdrawing:

side elevation a Fig. l'is a diagrammatic portion-of a paper'makingmachine to whichdevices constructed in accordance-with thisinventionhave been applied; 2 is an enlarged detail-sectionalviewshcwing the coating means; and

I'igJisanenlargedtransversesectionoithe doctor blade which is used incnilm ticn with the coating means.

Referring to the drawing in detail, this inven-:

.tion includes a doctor blade I which is preferably shaped from a bar-orplate of a metal such as stainless steeL- Oneedge of theblade Iismachine finished or ground to present a enerally quadricylindricalsurface 0, one end 0 which merges into the face 1 of the blade and theother end; otwhich terminates at an intersection with the opposite face8 of the blade. At the point of cylindrical surface 0 and, the face 8 oithe blade. the blade race is disintersection between the 5 around thelast upper so second dryer section.

' intermediate the dryer-sections A andB 15 eflectivetoapply coatingcompoaitionto oifthernovingwebofpaper. Astheweb installation thepaper-making ma-.chineinciudesaiirstdryersectioniiandaseconddryersectionnseparatedtherefrom.Coating apparatus generailyindicatcd at Cis as a timrii 5;

stage in order that the tactwiththedryerdrumsandthedrumsof aroundseveralof the I section and thence over another coating tus D, identical.with the coating apparatus C, by which coating composition is appliedto the posite face of the moving web. The web, after leaving the secondcoating apparahls, is passed over the remaining dryerdrumsahdthencetothecalender. stack and/or reel. 4 "lhecoatingapparatusateachoitheccatingstations C and D generally comprises a tank II .0! the paper-makingmachine with which the apparatus is'associated. A dip roll If isalsoposed normal to the tangent to the surface 0 at the point ofintersection, After the qu'adricylindrical surface I hasbeen formed onone edge oi the blade the outer surfaces of the blade are then broughtto a' relatively 18 finish. The blade is then subjected to some formortreatment by which a layer of material is deposited thereon to therebypresent a non-wettable surface. One manner of treatinmthe blade toobtain this result-is to electro-dep'osit a plating of metal, aschromiumor the-like and then polishing' the deposited layer with oilbearing polishing agents. The oil which serves as a vehicle tor thepolishingagents penetratesand fills the pores of the plating to producethereby a surface having a low aillnityfor aqueous coating compositions.Of course, the-high polish may be effected by other means and thentheplating may be impregnated with an oilsuch as mineral oil which isimmiscible insuch compositions to produce thereby the desirednon-wettable surface. It-is. of course.

entirely possible to employ various compositions elated withthe tank"and is partially immersed in the coating composition contained therein.

The dip roll I! is adapted to be constantlyrotated through power means,not shown, to thereby pick up a supply of coating material from the tankand present it to the web of paper which isdisposedinsurfacecontactwiththetopoi' the roll. Thus, an excess of coating compositionis. applied to the underside ofthe .web. After the web leaves contactwith the dip, roll it it is passedover the cylindrical suriace oi thedoctor blade l'where the excess coating is removed by a progressivelyincreasing but abruptly terminating wiping action and the coated surfaceis liven the desiredilnish. It should be noted that when the coated webof paper passes over the web-contacting portion ofthe doctor blade, theexcess of coating will immediately iiowback over the face I oi the bladeandwill not be carried beyond the intersection of the cylindricalsurface i with the plane surface I. Thus, there is no formation of apool of coating composition on the trailing side of the doctor bladebeyond the line of intersection between the curved and plane surfacesand the consequent stippling of the coating.

It should be noted that the best results in such as suitable blends ofwaxes and resins to" the finished applied coating are obtained whenistics.

the angle between the web on the trailing side of thedoctor blade andthe plane surface 8 thereof is maintained at approximately 90. Aneffective operating range has been found to lie within an angularity ofroughly 80 to 100. has been found that if this angle between the web andthe plane surface 8 i reduced much below 80 the applied coating willpresent a surface which is stippled or mottled in appearance,

or if the angularity between the web and the plane surface is increasedto much greater than 100 the applied coating is likely to present astreaked appearance. Of course it should be realized that the abovementioned effective operating angles are those which may be normallyencountered when the paper-making machine is being operated underreasonably uniform conditions as to the basis weight of the web, theamount of moisture remaining in the web at the time of coating, thetension of the web, the speed at which the web is traveling and theweight and composition of the coating to be applied. It

is quite likely that other variables may enter.

into the picture from time to time and will have to be givenconsideration in order to determine the most eflicient angulardisposition of the web relative to the trailing side of the doctorblade. Thus, if the furnish is greatly changed over that which had beenemployed, an initially satisfactory setup for the coating operation, orif the 'tension of the web is varied or if the speed of travel of theweb is varied, the setup of the coating apparatus will. have to bevaried in accordance with the new general web character- For the abovereasons, the angular values mentioned above should not be considered aslimiting, but are merely indicative of a range of angular values whichare effective under certain operating conditions.

'It has been found that the radius of curvature of the quadricylindricalsurface 8 of a doctor countered through the use of a paper-makingmachine in which this apparatus is employed, doctor blades having radiiof curvature varying from /10 of an inch to approximately one inch canbe satisfactorily employed.

The above-mentioned values for the angularity between the surface 8 ofthe doctor blade and the web, and the values for the range of effectiveradii of curvature have been predicated upon the use of a mineralcoating composition carried by an aqueous vehicle. It is not believed tobe necessary to herein describe any specific formula of coatingcomposition since such are well known. It should be noted that shouldcoating compositions be employed in which vehicles other than water areemployed to carry the mineral content, it is quite likely that theabove-mentioned ranges of values would become greatly modified accordingto the particular vehicle used and its effect relative tothenon-wetting'surface of the doctor blade, the radius of curvature of thedoctor blade and the angularity between the web and the normal face ofthe doctor blade.

In some instances when certain particular formulae of coatingcompositions are to be ap- 5 plied 'to.the web of paper, it may be founddesirable to pre-moisten that surface of the paper web to which thecoating is to be applied. In such instances the pre-moistening may beeffected through the use of a sweating or dampening roll l3 adapted totransfer moisture from its outer surface to the adjacent surface of theweb of paper passing th'ereover. The sweating roll I3 is preferablypower driven at a uniform rate of speedby means not shown and isjournaled adjacent the dip roll of the coating apparatus and is disposedin parallel alignment therewith. When the sweating roll is employed, themoving web of paper is first passed over the upper portion of thesurface thereof to thereby receive a certain amount of moisture and thenthe web is passed over the dip roll in the manner above described. Themoistening of the web prior to the actual application of the coatingcomposition thereto appears to facilitate uniform distribution of thecoating over the web. 4 It is believed that the above-stated descriptivmatter is set forth with sufficient clarity that the method of coating amoving web of paper in accordance with the principles of this inventionis adequately defined. To summarize, however, the method of coating acontinuously moving web of paper in accordance with this inventionconsists in applying an excess of coating composition to amoving web ofpaper, then passing the coated web over a doctor blade including anabruptly terminating wiping surface having a low aflinity for thecoating composition, whereby the excess of coating will be completelyremoved fromthe web while the web is in contact with the wiping surfaceand prior to passage of the web beyond the line of abrupt termination ofthe wiping surface to thereby produce a smoothly finished coating havingsubstantially no surface imperfectiens.

Thus, it will be seen that the herein-described invention discloses anovel doctor blade for use in conjunction with the coating of acontinuously moving web of paper, which includes a convex web-contactingportion on its leading side and terminating in an intersection with aplane surface disposed normal to the tangent at the intersection; inwhich the outer surface thereof possesses non-wetting characteristicswhereby flow of a coating composition over the intersection face will beprevented; and which provides a novel method of coating a continuouslymoving web of paper consisting in applying an excess 'of coatingcomposition to a moving web of paper, passing the thus coated web overan abruptly terminating wiping surface having a low affinity for thecoating composition, whereby the excess of coating will be completelyremoved from the web while the web is in contact with the wiping surfaceand prior to passage of the web beyond the line'of abrupt termination ofsaid surface to thereby produce a smoothly finished coating havingsubstantially no surface imperfections.

It is of course to be understood that certain details of arrangement andproportions of parts may be variously modified without exceeding thescope of the appended claims.

We claim:

1. A doctor blade for use in conjunction with paper coating maciinescomprising a body havbetween the curved surface and the plane suring asurface treated to present a lowaffinity for coating composition, saidsurface being formed on a convex web-contacting portion of a said blade,the radius of curvature of said webcontacting portion being not lessthan and not greater than 1", said web-contacting portion terminating inan intersection with a plane disposed radially to said web-contactingportion.

2. A doctor blade for use in conjunction with paper coating machinescomprising a body having a surface treated to present a low afllnity forcoating composition, said surface beingformed on a web-contactingportion of said blade, said web-contacting portion having a constantradius of curvature of not less than and not greater than 1", said webcontacting portion terminating in an intersection with a plane disposedradially to said web-contacting portion. i

3. Paper coating apparatus comprising a docposition, means to move a webof paper having an excess of said aqueous coating composition on asurface thereof progressively across said convex surface and across saidintersection in a plane at an angle of not more than 10 to the planetangent to said convex surface at said intersection, said doctor bladeexerting a constant- 1y increasing but abruptly terminating wipingpressure upon said coating. composition without wetting of said convexsurface by said coating composition whereby to remove a portion of saidcoating composition from said web. and to form on the remaining coatingcomposition a smooth surface free from stippling and streaking.

4. Paper coating apparatus comprising a metallic doctor blade havingformed thereon an arcuate surface having a radius of curvature between1;" and 1" and terminating in the intersection of said surface with aplane surface substantially radial of said arcuate surface, said arcuatesurfacebeing substantially non-wettable by an aqueous coatingcomposition, means to move a web of paper having an excess of aqueouscoating composition on a surface thereof progressively across saidarcuate surface and across .said intersection in a plane at an angle ofnot more than 10 to the plane tangent to said convex surface at saidintersection, said doctor blade exerting a constantly increasing butabruptly terminating wiping pressure upon said coating compositionwithout wetting of said arcuate surface by said coating compositionwhereby to remove a'portion of -said coating composition from said weband to form on the remaining coating composition a smooth surface freefrom stippling ands'treaking. J JOHN CAMPBELL.

KENNETH AUSTIN TAYLOR.

